There are two types of reactions:
The examiner may ask you the oxidation number of the compound.
First, let’s learn the basic rules of finding the oxidation number:
1. The atoms present in its element form have 0 oxidation number. Such as O2 or S.
2. For the mono-atomic ions, their ionic charge is their oxidation number.
Such as in NaCl, Na+, and Cl-
3. The formula unit of a compound has an oxidation number of 0.
4. Some compounds have a variable oxidation number in different compounds.
For example, iron in its free form is 0
While, when it forms a compound with chlorine; FeCl2 its oxidation number is +2, and FeCl3 its oxidation number is +3
5. The oxidation numbers of some of the groups are mentioned below:
Group (I): +1
Group (II): +2
Group (VI): -1
Hydrogen: +1 with non-metals
-1 with metals and boron
Oxygen: -1 in peroxides
-2 in all other compounds except fluorine
Q1: Determination of the oxidation number of carbon in CaCO3(s)
Ca= +2, O= -2 à O3= 3x(-2)= -6
As the oxidation number of the whole compound should be equals to 0, hence,
+2 + C + (-6) = 0
C = 0 – (+2) – (-6) = +4
A reaction in which both oxidation and reduction take place side by side is a redox reaction.
> An oxidized substance becomes a reducing agent for the substance that is being reduced.
> A reduced substance becomes an oxidizing agent for the substance that is being oxidized.
In the example above:
Methane is the reducing agent.
Oxygen is the oxidizing agent. It has a decrease in its oxidation number, but it has oxidized hydrogen.
It can also be seen that neither carbon nor hydrogen faces change in their oxidation number. However, because hydrogen and carbon both have formed bonds with oxygen, we say that they are being oxidized.
On the other hand, the oxygen, which initially had 0 oxidation number as it is a molecule, when binds with hydrogen and carbon, its oxidation number decreases. This shows that oxygen is being reduced.
A list of oxidizing and reducing agents:
|Chlorine ||Carbon monoxide|
|Concentrated sulfuric acid||Hydrogen|
|Nitric acid||Hydrogen sulfide|
|Potassium manganate (VII)||Potassium iodide|
|Potassium di-chromate (VI)||Sulfur dioxide|
|Hydrogen peroxide||Hydrogen peroxide|
• The examiner may give you a reaction in which you will be asked to tell which reactants are acting as reducing and oxidizing agents. Such example is mentioned below:
H2 + F2 à2HF
Example: In the reaction above, which reagent is the reducing agent, and which one is the oxidizing agent?
Answer. H2 is being oxidized, and it is reducing F2, which makes it a reducing agent.
F2 is being reduced, and it is oxidizing H2, which makes it an oxidizing agent.
• Test to check the Oxidizing Agents:
1. Potassium dichromate VI changes from orange to green precipitates.
2. Potassium manganate VII changes from purple to colorless.
3. Chlorine oxidizes the lesser reactive elements such as Iodine and bromine in a displacement reaction. The color of chlorine changes from green-yellow to colorless.
• Test to check the Reducing Agents:
1. Potassium iodide which changes color from colorless to brown
2. Carbon monoxide which reduces metal oxide to metal in the heat
3. Hydrogen reduces copper(II) oxide to copper
4. Sulfur dioxide is used as a bleach and in the preservation
5. Metals displace less reactive metals.
> There are other types of reactions besides redox reactions too. For example: